1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on paper in accordance with a signal corresponding to an image of an original read by a scanner and, more particularly, to the drive control of a scanner according to an image formation magnification.
2. Description of the Related Art
Recently, a copying machine called a digital copying machine has been developed. This copying machine reads an image of an original by a scanner unit, supplies a signal corresponding to the read original image to a thermal head, and transfers an ink on an ink ribbon onto paper by the thermal head, thereby forming an image corresponding to the original image.
The scanner unit comprises a first carriage on which a light source for emitting light onto an original is arranged, a second carriage on which a plurality of mirrors for bending an optical path of light reflected from the original are arranged, a zoom lens for guiding light from the mirrors to a photoelectric converter and changing a magnification of an image to be formed, a mirror unit for guiding light transmitted through the zoom lens to the photoelectric converter and correcting an optical path length if the magnification changes, the photoelectric converter for receiving the light reflected from the original and converting the received light into an electrical signal, and a stepping motor for driving the first and second carriages.
With the above arrangement, in order to read an image of an original, the light source is turned on, and the first and second carriages are moved along an original table. Light reflected by the original is guided to the photoelectric converter sequentially through the mirrors on the second carriage, the zoom lens, and the mirror unit and converted into an electrical signal.
An enlargement magnification of the copying machine of this type was generally up to about 150%. In recent years, however, enlargement up to 400% can be performed. In order to perform copying at a magnification of 150% or more by this copying machine, a rotational speed of the stepping motor for driving the first and second carriages is decreased lower than a speed at a normal copying magnification (70% to 150%), thereby decreasing a speed of scanning an original. When the rotational speed of the stepping motor is decreased, however, vibration or noise of the stepping motor is undesirably increased.
When the rotational speed is low, therefore, a torque (driving force) of the stepping motor may be set such that no vibration or noise is produced. When the torque of the stepping motor is decreased, however, a desired torque cannot be obtained upon copying at a normal magnification.